Vaccines are designed to protect the individuals who receive them from infection with various disease-causing agents, such as viruses and bacteria. Widespread vaccination has only led to complete eradication of one virus—smallpox—but routine vaccination has successfully decreased the incidence of many once common diseases.

Part of the reason vaccines are so successful is because of community immunity or herd protection (see figure, below). It works like this. The more people in a community that get vaccinated, the fewer people who are susceptible to infection (those who remain unvaccinated because they are either too young or immune compromised). When a high enough percentage of people are immunized, the chain of infection for contagious diseases is broken and the spread of disease within the community is contained. Conversely, when immunization rates decline, there are more susceptible individuals and the effects of herd protection can break down, leading to an increase in the spread of disease. Measles is a good example of what can happen when herd protection breaks down.

The measles virus is a highly contagious, airborne pathogen that causes telltale red spots and complications that can include diarrhea, hearing loss, seizures, brain swelling, and pneumonia in about 30% of cases. Before a highly effective vaccine was developed in the 1960s, measles was a nearly ubiquitous childhood disease.

In 1971, a combination vaccine against measles, mumps, and rubella, the so-called MMR vaccine, was introduced, leading to a precipitous decline in all three diseases. Two doses of the combined MMR vaccine are estimated to be 97% to 99% effective at preventing infection with the measles virus. Vaccinating infants and children against measles led to a 75% drop in cases worldwide between 2000 and 2013. Yet because vaccination is far from universal, the measles virus is still responsible for about 145,700 deaths annually.

In the US, high vaccination rates, combined with good disease surveillance and rigorous control of outbreaks, resulted in measles being successfully eliminated in 2000, according to the US Centers for Disease Control and Prevention. But the country is once again in the midst of a measles outbreak involving 178 cases in 17 states. Several European countries are experiencing similar outbreaks. These outbreaks occur when vaccination rates decline and herd immunity breaks down.

Scientists can determine how high vaccination rates must be to establish herd protection by figuring out how quickly and efficiently the pathogen can move through a population. Because measles is so highly contagious (a single infected individual can spread the measles virus on average to 12 to 18 people), the threshold of vaccine coverage to maintain herd immunity is around 95%. By contrast, only about 80% to 85% of individuals in a community need be vaccinated to maintain herd immunity against polio. Measles immunization rates in the US, which are 92% and slipping due to a variety of factors including a growing anti-vaccine movement, are no longer high enough to achieve herd protection.

The situation in West Africa is even worse. The ongoing Ebola crisis in Guinea, Liberia, and Sierra Leone has led to an estimated 25% drop in childhood vaccinations between 2013 and 2014. Public health experts estimate that the disruption in health care services, which left as many as 1.1 million children unvaccinated over the past 18 months, has dramatically increased the number of individuals susceptible to measles. They estimate that this could lead to outbreaks of more than 200,000 cases—double what it was before inoculations waned—and as many as 16,000 deaths. For this reason, global public health organizations are now discussing the feasibility of implementing national vaccination campaigns in Ebola-affected countries.